In the wireless communications industry, a given service provider is often granted two or more non-contiguous or segregated frequency bands to be used for the wireless transmission and reception of RF communications channels. In order to minimize system hardware costs, it behooves such a provider to utilize a common receiver for the simultaneous reception and processing of signals within the segregated frequency bands. However, the wider the separation between the bands, the more complex the receiver typically becomes.
As shown in FIG. 1, a typical problem with spectrum allocation is that the segregated spectrums may be granted to a given provider with varying frequency gaps therebetween. In the United States, an "A" band provider transmitting and receiving frequency channels within the shown A, A' and A" bands has a 10 MHz discontinuity (835-845 MHz) between its first frequency block (the A and A" bands) and its second frequency block (the A' band). On the other hand, a "B" band provider using the B and B' frequency bands only has a 1.5 MHz separation (845-846.5 MHz) therebetween.
In digitization receiving systems, A/D converters are usually employed to digitize the modulated analog signals received on the communications channels. With these systems, the challenge of the designer is to capture the desired bands of frequencies while minimizing the sampling rate of the A/D converter, to thereby minimize the amount of data a digital signal processor (DSP) would have to process. This also allows for the use of lower sampling rate A/D converters, thereby having a positive impact on cost and performance. Concomitantly, it is desirable to minimize the amount of hardware required to achieve this goal.
If the desired frequency bands are separated by a small frequency gap or are contiguous, the achievement of the above stated goal would be relatively straightforward--simply, the A/D converter would just have to sample at slightly more than twice the bandwidth of the composite frequency bands (i.e., slightly more than the Nyquist sampling rate for the composite frequency bands). However, if the desired bands of frequencies are separated by a large gap, the A/D converter would have to sample at a high enough rate such that its Nyquist bandwidth encompasses the two bands of frequencies including the large gap. This approach is inefficient because the A/D converter is also sampling the frequency gap which contains useless information. It is therefore desirable to reduce that gap by moving the two non-contiguous bands closer together so that they will fit within a smaller Nyquist bandwidth. The Nyquist bandwidth would have some amount of margin around all sides of the two noncontiguous frequency bands to allow for filter roll-offs and interference from aliasing.
Conventionally, the frequency gap is reduced by using a down-conversion stage for each of the frequency bands to down-convert and to manipulate the placement of each frequency band at intermediate frequencies (IF) such that the frequency bands are closer together. For two frequency bands that require down-conversion, this will always mean a minimum of two mixer stages (i.e., two mixers and two local oscillators). Since mixer stages usually account for a substantial portion of a down-converter's cost, it would be advantageous to eliminate a mixer stage whenever possible. Also, multiple local oscillators create more difficulty in designing and laying out circuits because of the additional care which must be taken to shield and isolate the local oscillator signals.
Alternatively, if only one mixer stage is used, then separate A/D converters are needed to digitize each of the frequency bands independently. That is, the single mixer stage would downconvert both frequency bands to IF. Then the IF would be divided into two separate paths with each path bandpass filtering a different frequency band. The output of each path would then be digitized by separate A/D converters. This approach adds another A/D converter which increases the complexity of the design as well as the cost. Moreover, steps must be taken to ensure adequate isolation of the two A/D converters, inasmuch as their spurious outputs must be contained.